1. Field of the Invention
The present invention relates to a deterioration determining device and method for an exhaust emission reduction device including two catalysts serially disposed in an exhaust passage of an internal combustion engine, for purifying exhaust gases, the deterioration determining device and method being configured to determine deterioration of a downstream one of the catalysts, and an engine control unit.
2. Description of the Related Art
Conventionally, the present assignee has already proposed a deterioration determination device for an exhaust emission reduction device in Japanese Laid-Open Patent Publication (Kokai) No. 2000-328929. This exhaust emission reduction device is comprised of a three-way catalyst disposed in an exhaust passage of an internal combustion engine at a predetermined location thereof, for purifying unburned fuel in exhaust gases, and a NOx purifying catalyst disposed downstream of the three-way catalyst, for purifying NOx in the exhaust gases. Further, the deterioration determination device is for determining deterioration of the NOx purifying catalyst, and is comprised of a LAF sensor disposed in the exhaust passage at a location upstream of the three-way catalyst, and an upstream O2 sensor disposed in the exhaust passage at a location between the three-way catalyst and the NOx purifying catalyst, and a downstream O2 sensor disposed in the exhaust passage at a location downstream of the NOx purifying catalyst. The three sensors are all for detecting states of the air-fuel ratio of exhaust gases.
In this deterioration determination device, as shown in FIGS. 4 and 5 of the Japanese Laid-Open Patent Publication (Kokai) No. 2000-328929, deterioration determination on the NOx purifying device is carried out based on an output value VLAF from the LAF sensor, an output value SVO2 from the upstream O2 sensor, and an output value TVO2 from the downstream O2 sensor. That is, on condition that the answer to the question of a step 43 is affirmative (YES) which means that the engine is in rich operation, a time period from a time point at which the condition of VLAF>VLAFREF is satisfied to a time point at which the condition of SVO2>SVO2REF is satisfied is counted as a first timer count value tmMON1 (steps 46 to 51). Further, a time period after the time point at which the condition of SVO2>SVO2REF is satisfied to a time point at which the condition of TVO2>TVO2REF is satisfied is counted as a second timer count value tmMON2 (steps 52 to 55). Then, according to the first timer count value tmMON2, a correction coefficient KMNO2 is calculated (step 66). Then, a value obtained by multiplying the second timer count value tmMON2 by the correction coefficient KMNO2 is set as a corrected count value tmMON2C (step 67), and when the corrected count value tmMON2C is smaller than a reference value TNOXREF, it is determined that the NOx purifying catalyst is deteriorated. In the other cases, it is determined that the NOx purifying catalyst is normal (steps 68 to 70).
As described above, according to this deterioration determination device, the deterioration determination on the NOx purifying catalyst is executed based on a result of comparison between the value obtained by correcting the second timer count value tmMON2 according to the first timer count value tmMON1 and the reference value TNOXREF. This method is employed because the first timer count value tmMON2 is calculated as a value representing the difference in timing in which the air-fuel ratio of exhaust gases on the upstream side of the three-way catalyst changes such that it becomes richer than the stoichiometric air-fuel ratio, and timing in which the air-fuel ratio of exhaust gases on the downstream side of the three-way catalyst changes such that it becomes richer than the stoichiometric air-fuel ratio, and hence it represents a degree of deterioration of the three-way catalyst.
According to the deterioration determination device proposed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-328929, the output values from the three sensors for detecting states of the air-fuel ratio of exhaust gases are directly used for determination of deterioration of the NOx purifying catalyst, and hence there is a possibility of lowering of the determination accuracy as described hereafter. For example, if the air-fuel ratio and/or exhaust gas flow rate changes due to a change in operating conditions of the engine, the reactivity of each of the two catalysts varies, which sometimes causes lowering of the accuracy of the determination. More specifically, since the first timer count value tmMON1 is calculated based on the output value VLAF from the LAF sensor and the output value SVO2 from the upstream O2 sensor, if the reactivity of three-way catalyst changes, this causes lowering of the accuracy of calculation of the first timer count value tmMON1, i.e. that of calculation of the correction coefficient KMNO2. Similarly, since the second timer count value tmMON2 is calculated based on the output value SVO2 from the upstream O2 sensor and the output value TVO2 from the downstream O2 sensor, if the reactivity of the NOx purifying catalyst changes, this causes lowering of the accuracy of calculation of the second timer count value tmMON2. As described above, if the accuracy of calculation of the correction coefficient KMNO2 and that of calculation of the second timer count value tmMON2 are lowered, the accuracy of the deterioration determination is also lowered accordingly.
Further, since the upstream O2 sensor and the downstream O2 sensor are disposed respectively between the three-way catalyst and the NOx purifying catalyst and on the downstream side of the NOx purifying catalyst, if the air-fuel ratio of exhaust gases is changed from the leaner side into the richer side, the timing in which the condition of SVO2>SVO2REF is satisfied and the timing in which the condition of TVO2>TVO2REF is satisfied change depending on the activities and sulfur-poisoned states of the three-way catalyst and the NOx purifying catalyst. This results in the lowering of the accuracy of calculation of the above two timer count values tmMON1 and tmMON2, and accordingly lowers the accuracy of the deterioration determination.